The invention relates to a casing for a steam or gas turbine, which comprises a shell and two flanges.
Compressor casings for gas turbines are known, which comprise a shell and two flanges. These casing have a pseudo-flange in the upper and lower region of the shell. These, however, have the disadvantage that increased radial expansion occurs in the region of the pseudo-flange during operating of the gas turbine owing to the higher mean temperature, and the casing thus changes from its round shape. This deformation leads to reduced efficiency, since the gap between the casing and the ends of the turbine blades is enlarged, and steam or air can flow through without any impediment at this point, without carrying out any work on the turbine.
Casings for steam turbines are also known, which comprise a shell and two flanges and which have vertical slits in the horizontal flanges. However, owing to the temperature distribution in the shell and the flanges, this leads to the shell being subjected to severe deformation as a result of the solid flanges being attached by screws. This deformation acts on the casing both radially and axially. Radially, an ellipsoid shape is produced from the round shell shape, since the shell expands upwards and the two flanges are moved slightly inwards. Axially, the radial effects have different effects within the casing owing to the different temperature distribution, and thus likewise lead to deformation. Owing to the necessary increased radial clearance between the casing and the ends of the turbine blades, this deformation leads to poorer efficiency, since steam can increasingly flow through, without carrying out any work on the turbine. The slits which the separating flanges have reduce in particular the axial deformation of the casing but, on their own, are not sufficient to prevent the radial deformation, and thus the reduced efficiency.
Furthermore, designs are known which use shrinking rings to prevent the deformation of a steam turbine casing. However, these designs have the disadvantage that they are very expensive and special assembly jigs are required for this purpose.
The object of the invention is to design a casing for a steam or gas turbine, which retains its round shape in operation or exhibits only relatively minor deformation, in order in this way to reduce the radial clearance between the casing and the ends of the turbine blades and to prevent the poorer efficiency associated with this. Furthermore, it is intended to avoid expensive designs and assembly jigs.
This object is achieved according to the invention in that the shell has different wall thicknesses in the upper region facing away from the flanges, in two central regions and in two lower regions facing the flanges, in which case the wall thickness of the upper region facing away from the flanges is reinforced in comparison with the wall thickness of the lower regions facing the flanges, and the wall thickness of the central regions is variable such that the upper region facing away from the flanges and the lower regions facing the flanges merge continuously into one another.
One advantage of this invention is that variation of the wall thickness of the shell of the casing considerably reduces deformation into an ellipsoid shape, both in the radial direction and in the axial direction. The casing thus has little radial clearance between the casing and the ends of the turbine blades and, in consequence, has considerably better efficiency than the prior art. Overall, the invention achieves an improvement in the steam turbine efficiency of 0.2% to 0.3%.
Further refinement options of the invention are the subject matter of the dependent claims.